The microbial quality of the working environment was assessed in winter in air-conditioned office buildings in Warsaw. The average indoor concentrations of bacterial and fungal aerosols were low (<10³ cfu·m⁻³), below Polish proposals for threshold limit values in public service buildings. Even during cold months, if the air-conditioning system works properly, people remain the main source of bacterial aerosol in offices, whereas infiltration of outdoor air remains a major mechanism responsible for their fungal contamination. An analysis of the bioaerosol size distribution showed that microbial propagules that reach both the upper and lower respiratory tract may evoke numerous adverse health effects from irritation and asthmatic reactions to allergic inflammation. A comparative analysis of viable and total airborne microbial counts showed that viable micro-organisms accounted for up to 0.3% of the total number of microbial propagules. Hence, a comprehensive hygienic assessment of office workplaces should include an efficient control of both these elements.